Solder connections are widely used in the manufacture of electronic devices such as printed circuit boards. For example, an integrated circuit (IC) is typically enclosed in a package including wire leads attached to the input and output portions of the IC and extending to the outside of the package. The terminal portions of the wire leads are coated with solder. Where the IC package is to be mounted, e.g., on a printed wiring board having formed on its surface an interconnective metal pattern, the metal pattern includes solder-coated contacts corresponding to the leads of the IC package.
To mount the IC package, solder paste, typically comprising solder particules suspended in a mixture of flux and binder, is applied to the printed wiring board. The leads of the IC package are aligned with the printed wiring board. A relatively small amount of force, typically a few thousand dynes, is applied to the entire package in order to force the leads into contact with the paste. The board is then heated to the melting temperature of the solder in a furnace, without any pressure being applied to the joints.
In an alternative procedure that is applied, for example, to packages having very fine pitch, the printed wiring board is reflowed, fluxed, and then a heated member, such as a horizontal bar, descends in a vertical direction, pressing down on, for example, a row of leads. The bar simultaneously heats the leads and applies a relatively small pressure to the leads.
Surface oxide is usually present on the solder-coated regions being joined, and more oxide usually forms during the application of heat (unless oxygen has been removed, excluded, or replaced by non-oxidizing gas near the pieces being joined). As a result of the presence of oxidized regions within the vicinity of the joint, solder joints are usually too mechanically weak and electrically resistive to be useful unless flux is applied to the surfaces that are to be joined.
However, for some applications, the use of flux is extremely undesirable. Flux is corrosive or capable of forming corrosive residues. The quantity of flux that is applied must often be kept within strict limits, because excess flux can cause beads or icidles of solder to form, which may cause short circuits, and paucity of flux may lead to the formation of brittle, porous joints.
Moreover, after the solder joint is made, it is often desirable to remove the flux. This has often been done by cleaning with chlorofluorocarbons (CFCs). Because CFCs have been found to be hazardous to the environment, it is desirable to reduce or eliminate their use. One way to reduce the use of CFCs is to eliminate the use of flux in soldering operations.
To avoid the problems associated with flux, various approaches have been attempted for joining metal parts by soldering, without flux, in a controlled atmosphere from which oxygen has been excluded. For example, a number of investigators have performed fluxless soldering in a reducing atmosphere comprising, for example, carbon monoxide or hydrogen. Although these and similar approaches have proven useful, reducing gases may be flammable or toxic, and therefore must be isolated from the ambient atmosphere by enclosing the parts to be joined within a sealed chamber.
In order to avoid the hazard and inconvenience of reducing atmospheres, various investigators have attempted fluxless soldering in inert atmospheres, comprising, for example, argon or nitrogen. For example, U.S. Pat. No. 3,900,151, issued to H. Schoer and W. Schultze on Aug. 19, 1975, describes a process for the fluxless soldering of aluminum-containing workpieces in a non-oxidizing atmosphere such as nitrogen, using zinc-based solder containing aluminum and one or more additional components. U.S. Pat. No. 4,615,952, issued to R. Knoll on Oct. 7,1986, also describes a zinc-based, aluminim-containing solder for the fluxless joining of aluminum workpieces in an inert atmosphere. U.S. Pat. No. 4,702,969, issued to B. Bunkoczy et al., on Oct. 27, 1987, describes a method of joining aluminum or titanium workpieces that have been precoated with solder by placing the surfaces to be joined in intimate contact and heating the workpieces in an inert fluorocarbon atmosphere. Bunkoczy mentions that no flux is required when an indium solder is used.
The solder compositions described in the above-mentioned patents are useful for joining aluminum or titanium workpieces, but they are different from the solder compositions commonly used for mounting semiconductor electronic devices. Various other investigators have attempted to perform soldering operations characteristic of the electronics industry without the use of flux by performing such operations in an inert atmosphere. For example, U.S. Pat. No. 3,705,457, issued to L. V. tardoskegyi on Dec. 12, 1972, discusses a fluxless wave-soldering technique in which the undersurface of a relatively flat workpiece is solder-coated by bringing it into contact with a stream of molten solder projected upwardly by a nozzle. A second nozzle discharges a stream of inert gas, such as nitrogen, which flows between the upper surface of the solder stream and the undersurface of the workpiece. Similarly, U.S. Pat. No. 4,821,947, issued to M. S. Nowotarski on Apr. 18, 1989, describes a wave-soldering technique for solder-coating metal surfaces, and for joining metal surfaces, in an inert atmosphere such as nitrogen. According to that technique, the surfaces to be joined are brought into contact with a solder bath.
A fluxless technique for soldering semiconductor chips is described in K. Mizuishi et al., "Fluxless and Substantially Voidless Soldering for Semiconductor Chips," IEEE 38th Components Conference Proceedings, May 1988, pp 330-334. According to this technique, a hollow solder preform is placed between a semiconductor chip and the surface it is to be mounted on. Before this assembly is heated, it is placed in a partial vacuum. During the heating stage, and while the solder is able to flow, the pressure is raised to. e.g., atmospheric pressure, causing molten solder that is essentially free of oxidation to flow into the central void of the preform.
For the purpose of mounting IC packages having wire leads, it is desirable to precoat the wire leads with solder, and also to precoat the surfaces to be joined to the wire leads. Furthermore, it is desirable to complete the joining step in the absence not only of flux, but also of added solder. Although Tardoskegyi and Nowotarski describe techniques that may be practiced using solder compositions that are conventional in the electronics industry, they do not address the problem of mounting precoated electronic components without adding more solder to the joint region. Because the technique described by Mizuishi relies, explicitly, on the use of a solder preform, it, too, fails to address the problem of mounting without additional solder.
Japanese Patent No. 62-81268, "Fluxless Soldering," issued in 1987 to Aoyama, Onuki, and Miyake, describes a method of precoating workpices with films of tin-silver alloy solder, and subsequently joining the workpieces without flux by heating them in a vacuum and placing the precoated surfaces in contact with each other with or without applying external pressure, and with or without adding solder between the precoated surfaces.
Significantly, the entire joining operation described by Aoyama (including the application of pressure, if used), must be performed in a vacuum. As a consequence, this approach suffers the inconvenience of requiring large vacuum systems.
Thus, until now, there has been no single technique for mounting solder precoated electronic components that is fluxless, uses solder compositions conventional in the electronic industry, may be performed in the presence of inert gas rather than in a vacuum, and requires no additional solder.